It’s a site so secretive that an underground railroad line delivers parts into a movable building, hiding activity from view.

North Korea describes it as a place where the country launches satellites into space.

But U.S. spy satellites are keeping a sharp eye on North Korea’s Sohae Satellite Launching Station at Tongchang-ri. And increased movement of equipment and personnel into the facility is raising renewed concerns in the U.S. military and intelligence community that something imminent could be in the offing.

Overhead satellite imagery in recent days has shown the movement of personnel, rocket-related equipment and fuel into the facility in the north of the country, according to a U.S. official.

North Korea will likely say it’s launching a satellite from the site, but U.S. officials say the rocket involved could be used as an intercontinental ballistic missile.

An unnamed Japanese government source said an analysis of satellite imagery indicates North Korea may be preparing for a missile launch, Japan’s Kyodo news agency said Thursday.

‘Provocation’

“Our military is prepared for various types of North Korea’s provocation and is … closely and continuously observing any signs of North Korea’s long-range missile launch,” Kim Min-seok, South Korean Defense Ministry spokesman, told reporters at a regularly-scheduled press conference.

If the launch does indeed transpire to be a missile launch, “It will be a direct violation of (a U.N. Security Council resolution) and … will be a severe provocation and threat against peace and stability of South Korea, northeast Asia and the world. It is our government’s official position that North Korea should not proceed with such provocative acts.”

Kim added that international law would oblige the North Koreans to declare a long-range rocket test.

“North Korea in the past had launched short-range missile without internationally declaring (a) navigation zone, but as for long-range missile they will have to internationally declare a no fly-zone.”

The suspected test buildup comes a day after U.S. Secretary of State John Kerry visited China, pressing the reclusive state’s closest ally to step up pressure on North Korea after a recent nuclear test.

Meanwhile, Japan’s Deputy Chief Cabinet Secretary, Koichi Hagiuda, said that his government would refrain from commenting on this specific case. He did say, however, that Japan would work with partners and keep pressure on North Korea to abide by the multilateral, “six-party” talks that aim to denuclearize the Korean peninsula.

“The government will closely cooperate with the U.S. and Korea and demand North Korea to restrain from provocative actions and to comply United Nations Security Council Resolution and the joint statement of six-party talks.”

Past attempts

A similar scenario unfolded in 2012, when North Korea announced it was launching a rocket carrying a satellite from the site.

North Korea said that operation was for peaceful purposes, but Japan, the United States and South Korea decried it as a cover for a long-range ballistic missile test.

Earlier this month, North Korea bragged about what it said was the “spectacular success” of its first hydrogen bomb test.

Experts have cast doubt on North Korea’s claim that it tested a hydrogen bomb. Among other things, the blast detected by seismic monitors was much smaller than would have been expected with a powerful hydrogen bomb, they say. But the move drew swift condemnation and calls for additional sanctions.